The use of spark plugs in internal combustion engines is well known. Generally, spark plugs have an inner conductor, an outer conductor, and an insulator between the two conductors. The spark plug threads into a threaded hole in the head of the internal combustion engine so that terminating ends of the first and second conductors extend into an enclosed area of the engine that contains fuel vapor so that a spark generated across a gap between the terminating ends ignites the vapor.
In use of spark plugs in racing engines, it is known to index a plug so that the gap between the terminating ends of the conductors opens at a desired angular position with respect to the spark plug's axis. Indexing plugs to desired positions is generally believed to increase engine horsepower. In general, indexing is achieved by placing washers above the spark plug thread so that the threaded connection between the spark plug and the engine head stops at a point corresponding to a desired angular position for the plug.
It is also known to remove a spark plug's outer thread so that the spark plug may be inserted into a quick-change sleeve that threads into the spark plug hole, for example as described in U.S. Pat. No. 5,839,403.